The present invention generally relates to magneto-optical recording/reproducing apparatuses and, more particularly, is directed to a magneto-optical recording/reproducing apparatus in which a signal is recorded by a magnetic field modulation system and which has an external magnetic field applying unit for applying an external magnetic field at the time of reproduction.
The magneto-optical recording and reproducing method has been proposed in which a laser light is irradiated on a recording medium to locally heat the medium to thereby form an information recording pit, that is, a magnetic domain therein and, the recorded information is read by magneto-optical interaction, that is, the Kerr effect or Faraday effect. According to this method, the size of the recording pit is required to be reduced in order to increase the recording density of the medium. In this respect, resolution of the reproduced information at the time of reproduction is determined on the basis of the wavelength .lambda. of a reproduction light and the numerical aperture N.A. of the objective lens of a reproduction optical system.
There has been made some proposals to improve the resolution of the reproduced information against the aforementioned limitation (hereinafter referred to as "super-resolution") as disclosed in Japanese Laid-Open Patent Publication No. 1-143042 and U.S. Pat. No. 5,218,581. In the proposals, a magneto-optical recording medium having a multilayer film structure constituted by a reproduction or playback layer, an intermediate layer (cutting layer) and a recording layer which are successively and magnetically coupled with one another at room temperature is used in the following manner. That is, upon reproduction, a laser beam serving as the reproduction light beam is irradiated on the recording medium to thereby temporarily extinguish or reduce the size of the magnetic domain in a high temperature portion formed in a laser beam spot but generate a magnetic domain serving as recording information only in a low temperature portion, thereby reading the information therefrom to thereby attain improvement of resolution at the time of reproduction and accordingly attain improvement of recording density.
FIGS. 1A and 1B are explanatory diagrams showing an example of such a magneto-optical recording system. FIG. 1A is a diagram typically showing a reading (reproducing) operation with respect to recording pits MP.sub.1, MP.sub.2, MP.sub.3, . . . FIG. 1B is a schematic sectional view of a magneto-optical recording medium 5 used in the system. The magneto-optical recording medium 5 is composed of at least a reproduction layer 51, an intermediate layer 53 and a recording layer 52. As the arrows given to the respective layers typically represent magnetic moments in the respective layers, for example, a downward magnetized condition is given as an initial condition. For example, informations of "0" and "1" are recorded as downward magnetization and upward magnetization respectively at least in the reproduction layer 51 and the recording layer 52. For example, information of "1" is recorded in the form of recording pits (magnetic domain) MP.sub.1, MP.sub.2, MP.sub.3, . . .
At the time of reading information from the recording pits, for example, an external magnetic field HR for reproduction (hereinafter referred to as "reproduction magnetic field") is applied in a direction along the direction of magnetization in the initialization or in the "0" recording condition of the reproduction layer 51 in order to attain a good carrier-to-noise ratio (C/N) and, at the same time, the reproduction light beam such as a laser light beam or the like is radiated onto a recording track of the magneto-optical recording medium 5 in which information has been recorded. In FIGS. 1A and 1B, SP represents a spot of the reproduction light beam on the magneto-optical recording medium 5. In this state, when the magneto-optical recording medium 5 moves from the right to the left direction in FIG. 1B as represented by an arrow a, temperature of a portion HT in the left side of the beam spot SP (that is, the front portion of the laser beam as viewed along the running direction of the medium) becomes higher due to the absorption of the reproduction light beam. On the other hand, the magnetic characteristics of the respective layers 51 to 53 of the magneto-optical recording medium 5 are selected so that, due to the high temperature of the portion HT, the coercive force of the reproduction layer 51 is lowered at the high temperature portion HT and that the temperature of the intermediate layer 53 in the high temperature portion HT reaches the Curie point thereof. That is, the recording pit remains as a latent image recording pit MP.sub.n only in the recording layer 52 when the pit is positioned in the high temperature portion HT even if positioned in the spot of the reproduction light, whereas recording pits in the reproduction layer 51 disappear or are extinguished, for example, by the reproduction magnetic field HR. As a result, the area in the reproduction beam spot SP except for the high temperature portion HT is obtained as an area from which recorded information can be read, that is, as a reproduction aperture AP, so that super-resolution in reproduction is thus attained.
The aforementioned super-resolution reproducing system employs such a system that magnetic domains are temporarily extinguished or reduced in the high temperature portion generated in the laser beam spot (hereinafter referred to as "extinction type reproducing system"). As disclosed in U.S. Pat. No. 5,168,482, for example, there has been proposed the so-called transfer type reproducing system in which recording pits in the reproduction layer are extinguished by the external magnetic field for initialization and at the same time the magnetic domain wall in the intermediate layer is extinguished only in the high temperature portion in the reproduction light beam spot to transfer recording pits of the recording layer to the reproduction layer to thereby make it possible to read information from the recording pits transferred onto the reproduction layer.
As a system for recording information, that is, for example, a system for rewriting previously recorded information on a magneto-optical recording medium to which such an extinction type or transfer type super-resolution reproducing system is applied, there has been proposed the so-called light modulation recording system. In the light modulation recording system, recorded information is once erased and then new information is recorded by applying a DC external magnetic field and by locally heating the recording medium by irradiating light beam modulated in accordance with the new information. This recording system, however, has a problem that a large time is required for erasing the recorded information.
As a measure to eliminate such a problem, there is generally known a method, that is, the so-called magnetic field modulation over-write (overlapped writing) method in which a magnetic field is modulated in accordance with information to be recorded at the time of recording.
According to the magnetic field modulation over-write method, laser light is emitted by a DC (direct current) or in synchronism with a signal clock pulse irrespective of the recording signal so that a high temperature portion generated in a medium by the laser light radiation is continuously kept in a recordable state. Under this condition, when an external magnetic field according to the information to be recorded, that is, modulated magnetic field is applied to the portion, new information can be recorded in the high temperature portion irrespective of the previously recorded information.
Accordingly, in the magneto-optical recording/reproducing apparatus using such a super-resolution reproducing system and using the magnetic field modulation system in which a recording medium is subjected to magnetic field modulation, the following arrangement as shown in FIG. 3 is performed. That is, in this arrangement, there is provided an optical pickup PC which irradiates a light beam such as a laser light beam or the like onto a magneto-optical recording medium 5 such as a magneto-optical disk with a predetermined light power at the time of recording and reproduction operation of information respectively. Further, there is provided an external magnetic field electromagnet 1 commonly used for recording and reproduction which applies a required reproduction external magnetic field such as an external magnetic field for initialization to the light beam irradiated portion and applies a modulation magnetic field according to information to be recorded to the portion at the time of recording of the information.
When recording is performed by using the magnetic field modulation system, the electromagnet 1 is required to be formed so that the inductance of a coil therein is set to be in a small value in order to attain good high-frequency response because the frequency used for recording is a high frequency of the order of several MHz.
This magnetic field modulation recording will be considered as follows. Assuming that the angular frequency used for recording is .omega. and the number of turns per unit length of the coil is n, the inductance L of the coil is given by EQU L=.mu.(.omega.)l Sn.sup.2 ( 1)
where .mu.(.omega.) is the permeability of a core material of the electromagnet, S is the sectional area of the coil, and l is the entire length of the coil.
The impedance of the coil is represented by .omega.L if the internal resistance thereof can be neglected.
When sinusoidal AC electromotive force E(t) with the recording angular frequency .omega. of from the order of several MHz to the order of several tens of MHz is given to the coil, the current I flowing in the coil is given by the following expression (2): ##EQU1## where e is natural logarithm, j is imaginary unit, r is the other impedance of a circuit containing the coil, and .delta. is a phase lag tan.sup.-1 (.omega.L/r).
That is to say, with respect to the applied AC electromotive force, the amplitude of the current depends on the angular frequency and the phase of the current lags by .delta.. Accordingly, when the AC electromotive force E(t) has a rectangular waveform with the frequency .omega..sub.0, E(t) is expressed by a Fourier series represented by the following expression (3): ##EQU2##
Therefore, the current I(t) flowing in the coil in accordance with the electromotive force E(t) is represented by the following expression (4): ##EQU3## where .delta..sub.n satisfies the relation .delta..sub.n =tan.sup.-1 (m.omega..sub.0 L/r) (m=1, 3, 5,..).
Accordingly, when (m.omega..sub.0 L/r).sup.2 and tan.sup.-1 (m.omega..sub.0 L/r) in the m-order component of I(t) are values which cannot be neglected, both the reduction of the amplitude and the phase lag are conspicuous in the component so that I(t) cannot be obtained as a rectangular wave with the frequency .omega..sub.0 because of the distortion of waveform. As a result, recording magnetic field H(t)=.mu.(.omega.)nI(t) cannot be well obtained. In order to maintain good high-frequency characteristic of H(t), it is necessary that the inductance L of the coil is reduced, that is, the number of turns n in the coil is reduced so that .omega..sub.0 L is smaller relative to r. On the contrary, in order to obtain sufficient reproduction magnetic field at the time of reproduction by using the same electromagnet as used at the time of recording, the stationary current I flowing in the coil must be increased because the value of L is small, that is, because the value of n is small. Accordingly, as long as the same electromagnet is used both at the time of recording and reproduction, electric power consumed at the time of reproduction cannot be saved if magnetic field modulation recording using a high frequency is guaranteed. In view of the specification of the magneto-optical recording medium, it is considered that the time used for reproduction is remarkably longer than the time used for recording. It is therefore very important that electric power consumed at the time of reproduction is saved.